This invention encompasses control of combustion oscillations in lean premix (LPM) combustion systems, i.e., systems in which the volume of fuel present is less than the stoichiometric ratio with the air. More particularly, this invention relates to the apparatus and method for reducing undesirably high pressure oscillations in a LPM combustion chamber to acceptably lower levels by using natural combustion and nozzle acoustics to generate multiple fuel pockets which counteract the oscillations caused by variations in heat release. This apparatus and method of operation represent a hybrid of active and passive control technology and are capable of eliminating combustion oscillations over a wide combustor operating range. The apparatus requires no moving parts or electronic control systems, as do most active control techniques. Also, the apparatus and method of operation are applicable to any lean premix combustion system operating on any gaseous or liquid hydrocarbon fuel.
Combustion systems such as used in conjunction with gas turbines and steam-generators commonly use a hydrocarbon fuel with air in substantially stoichiometric ratios in an associated combustion chamber for the generation of sufficient heat energy for driving a turbine or for the generation of steam. However, the burning of hydrocarbon fuels in these systems produces environmental pollutants such as nitrogen oxides, which are tightly regulated. Efforts to reduce the production of these environmental pollutants include pre-mixing the fuel and air prior to introducing the mixture into the combustion chamber. Increasingly lean premixes, i.e., a mixture where the volume of fuel present is less than the stoichiometric ratio with the air, are used to avoid the high temperatures that produce thermal nitrogen oxides. A typical combustion system using a lean premix (LPM) is described in U.S. Pat. No. 5,372,008, which issued on Dec. 13, 1994, and which is incorporated herein by reference.
While LPM combustion has been successful in reducing the emissions of environmental pollutants, it is prone to combustion instability in the form of dynamic pressure oscillations. Pressure oscillations can occur when variations in heat release due to fuel feed variation periodically couple to acoustic modes in the combustion chamber. Oscillations can be thought of as a closed loop interaction between combustion and acoustic processes. A variation in heat release produces an acoustic disturbance that is reflected by the combustor walls, thus resulting in a pressure disturbance. Although the magnitude of this pressure disturbance is reduced by acoustic losses, it may produce some change in the combustion process, thereby altering the heat release, and closing the feedback loop. With the correct timing of the feedback, and sufficiently small losses, the oscillation magnitude can grow to a limit cycle or steady oscillation, meaning that the amplitude of the oscillations does not change.
As indicated by Rayleigh's criteria, "Theory of Sound," Volume II, No. 8, Dover, N.Y., 1945, the amplitude of the oscillations in the combustion chamber will be the strongest when the pressure wave is in-phase with the periodic heat release produced by the combustion of the fuel-air mixture. These dynamic pressure oscillations are frequently of a sufficiently high magnitude as to produce undesirable operating conditions including reducing the useful life of the combustion system components due to structural fatigue, vibrations and cycling fatigue.
Past research efforts have resulted in various passive and active control techniques designed to remove or reduce pressure oscillations. These efforts have achieved varying levels of success. Active control techniques typically involve the elimination of instabilities via the release of energy which is out of phase with the pressure oscillation. The disadvantage of active control schemes is that they generally involve complicated control equipment. Passive techniques typically increase acoustic losses or shift instability regions to a different part of the operating envelope via geometric alterations. The disadvantage of passive control schemes is that they may introduce efficiency losses (i.e., increased nozzle pressure drop), they may be reliable only at select conditions, or they may create new instability problems at different operating conditions.
A recent development found to satisfactorily suppress high-amplitude pressure oscillations in hydrocarbon-fueled combustion systems is described in applicant's copending patent application titled "Combustor Oscillating Pressure Stabilization and Method," Randall S. Gemmen, et al, Ser. No. 08/644,609, filed Apr. 26, 1996. In this copending patent application, the active control of unsteady combustion induced oscillations in a LPM combustion chamber is provided by restructuring and moving the position of the main flame front to increase the transport time and displace the pressure wave further out of phase with the periodic heat release. The restructuring and the repositioning of the main flame front are achieved by utilizing a pilot flame which is pulsed at a predetermined frequency corresponding to less than about one-half the frequency of the combustion oscillation frequency. The duration of each pilot-flame pulse is sufficient to produce adequate secondary thermal energy to restructure the main flame and thereby decouple the heat release from the acoustic coupling so as to lead to a reduction in the dynamic pressure oscillation amplitude. The pulsating pilot flame produces a relatively small and intermittent flame front in the combustion zone that is separate from the oscillating main flame front. This provides sufficient thermal energy to effectively reposition the oscillating main flame front from the region in the combustion zone where acoustic coupling can occur to a region of reduced coupling, thereby effectively altering the oscillation-causing phase relationship with the heat of combustion. This copending patent application is incorporated herein by reference.
Another recent development found to satisfactorily suppress high-amplitude pressure oscillations in hydrocarbon-fueled LPM combustion systems is described in applicant's copending patent application titled "Combustor Oscillation Attenuation Via the Control of Fuel-Supply Line Dynamics," George A. Richards and Randall S. Gemmen, Ser. No. 08/744,644, filed Nov. 6, 1996. In this application, the active control of unsteady combustion induced oscillations in a LPM combustion chamber is provided by utilizing an acoustically tunable fuel-delivery system for promoting a greater heat release in the combustion chamber during each low pressure segment of the pressure oscillations. Fuel-rich regions of combustible fuel and oxidizer mixtures are provided at the flame front of the combustion oscillation at a time when the pressure oscillation is at the lowest segment. The burning of these fuel-rich regions produces a relatively large heat release at the pressure minima, thus resulting in significant attenuation of the pressure oscillations. Generally, the goal of this invention is achieved by acoustically tuning either the main fuel supply line or the pilot fuel supply line so that each pressure wave generated in the main combustion chamber creates an oscillation in the fuel contained in that supply line, thereby effectively creating a fuel-rich region at the supply line exit or fuel injection nozzle of the fuel-supply line that will be transported to and arrive at the flame front at a time when the pressure in the combustion chamber produced by oscillation is in a low, preferably the lowest, pressure phase. This tuning may be accomplished through the use of movable parts such as by forming a portion of the fuel-delivery line from two tubes, one placed inside the other with a section of the second tube being telescopically movable within the first tube so as to alter the length of the fuel-delivery line, thereby changing the phase of the oscillations in the fuel-delivery line. This copending patent application is incorporated herein by reference.
While active control techniques for reducing or suppressing undesirable pressure oscillations in LPM combustion systems fired with a suitable hydrocarbon fuel and oxidizer described above and in the publications referenced in applicant's aforementioned copending patent applications may satisfactorily reduce combustion oscillations, the primary objective or goal of the present invention is to combine active and passive control techniques to provide a further and improved apparatus and method for reducing combustion pressure oscillations in combustion chambers.
A further objective of the invention is to provide a simplistic approach to reducing pressure oscillations that requires no moving parts or electronics, unlike most active combustion control techniques. This is accomplished by using the natural combustor/nozzle acoustics in the combustion system to generate fuel pockets that counteract oscillations in heat release. As in an active control system, fuel pockets are deliberately generated to produce the stabilizing effect. In contrast to an active control system, however, the fuel pockets are not generated by mechanical means. Instead, the fuel system creates pockets via a dynamic response to the combustor pressure oscillations. With the correct distribution, these injected fuel pockets can stabilize an oscillation.
Other and further objectives of the present invention will become obvious upon an understanding of the illustrative embodiment and method about to be described or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon using the invention in practice.